WASHINGTON (CNN) -- Violent animal rights extremists and eco-terrorists now pose one of the most serious terrorism threats to the nation, top federal law enforcement officials say.

Senior officials from the FBI and the Bureau of Alcohol, Tobacco, Firearms (ATF) and Explosives told a Senate panel Wednesday of their growing concern over these groups.

Of particular concern are the Animal Liberation Front (ALF) and the Earth Liberation Front (ELF).

John Lewis, the FBI's deputy assistant director for counterterrorism, said animal and environmental rights extremists have claimed credit for more than 1,200 criminal incidents since 1990. The FBI has 150 pending investigations associated with animal rights or eco-terrorist activities, and ATF officials say they have opened 58 investigations in the past six years related to violence attributed to the ELF and ALF.

In the same period violence from groups like the Ku Klux Klan and anti-abortion extremists have declined, Lewis said.

The ELF has been linked to fires set at sport utility vehicle dealerships and construction sites in various states, while the ALF has been blamed for arson and bombings against animal research labs and the pharmaceutical and cosmetics industry.

No deaths have been blamed on attacks by those groups so far, but the attacks have increased in frequency and size, said Lewis.

"Plainly, I think we're lucky. Once you set one of these fires they can go way out of control," Lewis said.

"The most worrisome trend to law enforcement and private industry alike has been the increase in willingness by these movements to resort to the use of incendiary and explosive devices," he said.

The FBI also identified a British-based group, Stop Huntingdon Animal Cruelty, as a U.S. terror threat. The group targets Britain's Huntingdon Life Sciences Laboratory, which has an American facility in East Millstone, New Jersey.

Last year a federal grand jury indicted seven people identified as members of the group on charges they vandalized company property and harassed lab employees and customers.

Inhofe alleges PETA link

Senate Environment Committee Chairman James Inhofe estimated the cost of damages from militant environmental and animal rights supporters at more than $110 million in the past decade.

"Just like al Qaeda or any other terrorist movement, ELF and ALF cannot accomplish their goals without money, membership and the media," the Republican senator from Oklahoma said.

Inhofe said there was "a growing network of support for extremists like ELF and ALF," and he singled out People for the Ethical Treatment of Animals for giving money to members of both groups.

PETA claims more than 800,000 members. Its president, Ingrid Newkirk, declined to appear at the hearing, but general counsel Jeffrey Kerr denied Inhofe's allegation in a written statement.

"PETA has no involvement with alleged ALF or ELF actions. PETA does not support terrorism. PETA does not support violence," Kerr said.

Skepticism from some

Some committee members have expressed skepticism over the high level of concern toward environmental and animal rights extremists.

"The Department of Homeland Security spends over $40 billion a year to protect the home front," Sen. Frank Lautenberg said. After listing al Qaeda, Hamas and Hezbollah, the Democrat from New Jersey wanted to know who else the law enforcement agencies considered terrorists: "Right to Life? Sierra Club?"

Lautenberg declared himself "a tree hugger."

And Sen. James Jeffords also issued a statement expressing doubt about the target of concern.

"Congress can't do much about individual extremists committing crimes in the name of ELF or ALF, but we can act to significantly enhance the safety of communities across the nation," the independent from Vermont wrote.

"ELF and ALF may threaten dozens of people each year, but an incident at a chemical, nuclear or wastewater facility would threaten tens of thousands."